Replaceable gang head machine tools have been proposed which include a machine base provided thereon with a working unit, plural gang heads on the periphery thereof and an index table on the upper side thereof for selecting any desired one of the gang heads. It is so operated that any desired one of the gang heads is selected to be positioned in front of the front side of the working unit and then moves forward together with the unit, and a working operation is given to a workpiece positioned in front thereof. For example, one such machine is disclosed in U.S. Pat. No. 4,216,572. It is desirable in this type of machine tool that the gang heads on the machine tool should be replaceable with other additional gang heads stocked outside the machine tool ready to give any different working operation on a workpiece. For meeting this desire, there has been hitherto proposed a stock arrangement wherein the machine tool constructed as above is provided on its lateral outside with a stockyard extending long along the same.
Plural gang heads are stocked therein so that the gang heads on the stockyard side and the gang heads on the machine tool side may be replaced one with another through an intermediate carrier interposed between the machine tool and the stockyard. For example, one such system is disclosed in U.S. patent application Ser. No. 95,556 filed on Nov. 19, 1979. This type of arrangement, however, is not always favorable because a comparatively large wide space is required on the lateral side of the machine tool, for providing or installing the stockyard and consequently the floor space needed for the machine tool unit becomes comparatively large in width.